Prosthesis with biologically inert wear resistant surface

ABSTRACT

A prosthesis is provided having at least a smooth non-articulating load bearing surface disposed adjacent a bone. The prosthesis includes a substrate formed from a metallic alloy. At least the regions of the substrate defining the load bearing surface are coated with a biologically inert abrasion resistant material harder than the substrate for preventing leaching of ions from the substrate into adjacent body tissue and for preventing wear. The coating may be titanium nitride or zirconium or other such material exhibiting biological inertness and acceptable hardness. The coating preferably defines a thickness of 8-10 microns.

RELATED APPLICATIONS

This application is a continuation of Ser. No. 08/161,982, filed on Dec.02, 1993, now abandoned, which is a continuation of application Ser. No.07/882,256, filed on May 13, 1992, now abandoned, which is acontinuation-in-part of application Ser. No. 07/583,459, filed on Sep.17, 1990, now abandoned.

BACKGROUND OF THE INVENTION

Prior art orthopedic prostheses are used to replace or supplement aportion of the natural bone. Most orthopedic prostheses include at leastone surface region intended for secure fixation to the natural bone.Orthopedic prostheses may also include a load bearing surface regionthat is disposed adjacent to the bone without secure affixation. Manyorthopedic prostheses replace a natural joint, such as a knee, hip,shoulder, finger or ankle. Prosthetic joint replacement systems willinclude two prosthetic components affixed to separate bones and capableof articulating relative to one another. Thus, a component of aprosthetic joint may include an articulating surface region, a loadbearing non-articulating surface region and a surface region for directaffixation to adjacent bone.

Prior art prosthetic systems have employed screws, wedge fitting andcement either separately or in various combinations to fix theprosthetic device to the bone. However, cement is known to at leastpartly deteriorate after an extended time in the body, thereby causing ashifting of the prior art prosthetic device relative to the bone. Thismovement abrades the cement, the bone and the prosthetic device,resulting in release of wear debris which produces bone death andfurther loosening. Prosthetic devices relying entirely on screws orwedge fitting also may shift over time in response to changes in thenatural bone and/or forces exerted on the prosthesis, and abrasionresulting from such shifting generates wear debris as explained above.

The more recent prior art has included prosthetic devices with surfacecoatings or modifications that are intended to enhance natural boneingrowth to the prosthesis for achieving a biological fixation of theprosthesis to the natural bone. More particularly, at least selectedsurface areas of these prior art prostheses are provided with a macrosurface defining pores, fissures, texturing or the like into which thenatural bone tissue may grow. This biological fixation is intended tostabilize the prosthetic device relative to the bone and substantiallyprevent the loosening or shifting which had occurred with the abovedescribed earlier fixation means.

The surface treatment to enhance the bone ingrowth to the prior artprosthetic device can be achieved in many ways. A common surfacetreatment for prior art prosthetic devices is referred to as a porouscoating which is applied to the metallic substrate of the prosthesis todefine an array of small pores or fissures in the surface of theprosthesis. Other prior art prosthetic systems apply a metallic meshmaterial to a substrate, such that the mesh material defines the textureinto which bone tissue may grow. Still other prior art systems directlymodify the substrate to define the surface irregularities, such as smallholes, fissures, slots or the like.

Orthopedic prosthetic devices typically are formed from a metallic alloythat will exhibit appropriate strength and flexure in use. Examples ofmetallic alloys that are currently used for orthopedic prostheticdevices include titanium alloys, such as a titanium aluminum vanadiumalloy, and cobalt-chromium alloys, such as a cobalt-chromium molybdenumalloy. Although both titanium and cobalt-chromium alloys exhibitappropriate strength and flexure for most applications, each such alloyhas its own unique advantages and deficiencies. For example,cobalt-chromium exhibits desireable hardness, and hence is widely usedfor prosthetic devices having an articulating surface region. However,cobalt-chromium is very expensive and its desireable hardnesscharacteristics make it difficult to machine. Furthermore, some patientsexhibit sensitivity to cobalt-chromium alloys. Additionally, somecobalt-chromium alloys are known to release metallic ions as a result ofcorrosion after extended exposure to the biological tissue of the humanbody. These ions are suspected to cause tumors and may have carcinogeniceffects.

The amount of ion released from the alloys defining the substrate ofprosthetic devices generally has been considered acceptably low.However, it is known that the amount of ion release increases with thesurface area of the prosthesis. The relatively recent advent of surfacecoatings or modifications to promote bone ingrowth results in a verysubstantial increase in surface area. More particularly, pores, fissuresor other such surface irregularities vastly increase the surface areafor a given prosthesis as compared to the same prosthesis having asmooth exterior surface. This increase in surface area achieved by thepores, texturing or other surface irregularities has resulted in anincrease in the ion release from the alloy from which the prosthesis ismade. The ions released from the alloy of the prior art prostheticdevice may leach into the body and migrate to areas remote from theprosthetic device. Patients having prostheses with surface treatmentsfor promoting bone ingrowth have been observed to have ions in urinespecimens, liver tissue, and other body locations remote from theprosthesis.

In view of these fairly recent findings, some doctors recommend notemploying prostheses with macro surface treatments in young patients whomay be expected to be exposed to the large surface area of ion releasingalloys for a number of years and who therefor run a greater risk ofbeing adversely impacted by the tumor causing or carcinogenic effects ofthe ions.

In addition to the preceding negative effects of ion release from priorart prosthesis having surface treatments for promoting bone ingrowththere is a desire to improve the biological fixation between theprosthesis and the natural bone. The ingrowth of bone to the macrosurface region of the prior art prosthesis often is not complete. Datasuggest that bone will grow into only about 10-20% of many prior artmacro surface area. Furthermore, a thin fiber layer may exist betweenthe substantially rigid bone and the pores, fissures or otherirregularities of some the prior art macro surfaces. This relativelyincomplete bone ingrowth may be due to the above described corrosionreaction and resulting ion release. The effect of the incomplete boneingrowth may be some movement between the prosthesis and the bone. Asnoted above, such movement will generate metallic wear debris. The smallmetallic particles produced by such wear corrode rapidly in view of therelatively large surface area to volume ratio for these particles. Asnoted above, corrosion results in the undesirable metallic ion release.Thus a more complete biological fixation could reduce the potentiallyharmful metallic ion release.

Titanium alloy prostheses generally are considered to be much morebiologically compatible than the cobalt-chromium alloys. Thus, thesensitivity some patients have to cobalt-chromium prostheses generallyis not a problem for titanium alloy prosthetic components. Titaniumalloys also are substantially less expensive than cobalt-chromiumalloys, but they are not as hard. Thus, titanium alloys prostheticcomponents are likely to generate wear debris when employed onarticulating surface regions of a prosthetic component or on anon-articulating load bearing surface region that is subject tomicromovement relative to adjacent bone. The metallic wear debrisparticles cause further deterioration of both the prosthetic componentand the natural bone.

The prior art has included prosthetic components formed from a pluralityof different metallic alloys. For example, femoral prosthesis have beenprovided with a titanium alloy stem and neck and a cobalt-chromium alloyhead. The comparatively harder cobalt-chromium head performs well as anarticulating surface. The titanium alloy stem and neck achieve betterbiocompatibility at a lower cost. However, galvanic action is known tobe generated at the interface of the titanium alloy neck and thecobalt-chromium alloy head, with a corresponding corrosion andgeneration of corrosion-related debris. Additionally, load bearing areasof the stem are subject to micromovement relative to the bone, and hencethese areas can generate significant wear debris. Furthermore, it wouldbe desireable to provide a less expensive, harder and more biologicallycompatible head than the cobalt-chromium head in the prior art system.

The prior art also has included attempts to provide coatings on ametallic alloy prosthesis to enhance some aspect of its performance. Thecoatings have included ceramics which are noted for their hardness andtheir biological compatibility. These ceramic coatings have been appliedto the metallic alloy substrate by known thin film technology. Thistechnology is widely used, for example, in the machine tool arts toenhance the life of cutting tools. Prior art thin film technologytypically applies the ceramic coating to a substrate in a vacuum coatingchamber. The substrate to be coated functions as the cathode in thechamber, while the anode is formed from the material to be coated ontothe substrate. An arc is struck in the chamber, and the substrate to becoated is subjected to high energy ion bombardment from the anode. A gasis then introduced into the chamber. The gas reacts with the ions of theanode and produces an ionic deposition of a highly-adherent ceramiccoating onto the substrate. This thin film technology typically isemployed to provide an ionically bonded coating approximately 2-4microns thick. Thicker ceramic coatings have been considered too costlyand problematic for application by thin film technology, and hence thinfilm technology has not been employed for thicker coatings in themachine tool art and have not been carried over into the orthopedicprosthetic art. The problems of using thin film technology to produce athick coating have been cracking and eventual delamination. It has beenbelieved that cracking occurs in part due to the different stiffnessesof the metallic alloy substrate and the ceramic coating. Thus, ceramiccoatings applied by thin film technology on prosthetic devices generallyhave been in the range of 2-4 microns thick on articulating surfaces toavoid delamination and to minimize coating costs. Ceramic coatingsapplied by thin film technology to non-articulating surfaces areintended primarily to achieve biological compatibility, and hence havebeen at the lower end of this 2-4 micron range of coating thicknesses.Ceramic coatings have been applied to greater thickness by other coatingtechnologies (e.g. plasma spray) primarily for other art areas, such askitchen appliances. However, thick ceramic coatings applied by othertechnologies are also subject to cracking and delamination.

Despite the known hardness and wear resistance of thin film ceramicmaterials, the inventors herein have determined that thin film ceramiccoatings applied to articulating surfaces tend to wear through wellwithin the anticipated life of the patient and the prostheses. Thewearing through of the prior art ceramic coating exposes the prior artmetallic alloy substrate with the above described problems of weardebris, corrosion and biological incompatibility. The inventor's hereinfurther believe that similar wear through occurs at other load bearingnon-articulating surfaces due to the above described micromovementbetween the coated prosthesis and adjacent bone.

In view of the above, it is an object of the subject invention toprovide an orthopedic prosthesis with enhanced wear resistance.

It is another object of the subject invention to provide an orthopedicprosthesis that ensures both wear resistance and biologicalcompatibility.

A further object of the subject invention is to provide an orthopedicprosthesis that avoids galvanic corrosion between dissimilar metallicalloys of a prosthetic system.

Yet another object of the subject invention is to provide a coatedorthopedic prosthesis that avoids cracking and delamination in use.

SUMMARY OF THE INVENTION

The subject invention is directed to an orthopedic prosthesis. Theprosthesis of the subject invention comprises a substrate formed from ametallic alloy selected to exhibit appropriate strength, flexibility andweight characteristics. Examples of such alloys may include a titaniumalloy, such as a titanium aluminum vanadium alloy, or a cobalt-chromiumalloy, such as a cobalt-chromium molybdenum alloy.

The orthopedic prosthesis of the subject invention includes at least oneload bearing surface for transferring an applied load to the naturalbone. At least selected portions of the surface of the prosthesis may becoated or modified to enhance bone ingrowth and hence achieve biologicalfixation to the bone in which the prosthetic device is implanted. Moreparticularly, at least selected surface areas may be appropriatelytreated to define surface irregularities or texturing. The surfaceirregularities may be in the form of pores, fissures, holes, slots orthe like defining small regions into which bone tissue may grow forachieving a biological fixation of the prosthetic device to the bone.The surface treatment may be defined by a coating applied to theprosthesis or by appropriate modification of the substrate of theprosthesis. The pores, fissures or other such surface irregularitieseach preferably are in the range of 150-500 microns across. However, atleast selected load bearing surfaces of the prosthesis may be smooth toprevent bone ingrowth. The prosthesis may further include anarticulating surface region for articulation relative to anothercomponent in a prosthetic system.

The prosthesis of the subject invention further comprises a coating of amaterial that is harder and more abrasion resistant than the metallicalloy substrate of the prosthesis. The coating is applied by thin filmcoating technology to achieve ionic bonding at least to areas of theprosthesis that are subject to movement relative to an adjacent surface.For example, the coating may be applied to a relatively smooth loadbearing surface of the prosthesis that is subject to micromovementrelative to adjacent surfaces of the bone. The hard abrasion resistantcoating may also be applied to articulating surfaces of the prosthesis.

The hard abrasion resistant coating also preferably is biologicallyinert and may be applied to areas of the prosthesis having the surfacetreatment for promoting bone ingrowth. The relatively inert coating isapplied in a manner to maintain the overall surface irregularities onthe prosthesis for promoting bone ingrowth. Furthermore, the coating isapplied with a thickness that will keep the surface irregularities inthe above described range of 150-500 microns for optimizing boneingrowth. Thus, the surface regions of the prothesis that are treated toachieve bone ingrowth are sealed to prevent ion release and leachingdespite the substantial increase in surface area for these regions ofthe prosthesis.

A preferred hard biologically inert coating material, as explainedfurther below, is titanium nitride. Alternate coating materials arezirconium, titanium boride, titanium carbide, aluminum oxide anddiamond. These materials have been demonstrated to be substantiallycompletely biologically inert even when exposed for extended periods oftime and even with the very large surface areas inherent in regions ofthe prothesis that are treated as described above for promoting boneingrowth. Furthermore, these coating materials exhibit desirablehardness characteristics for preventing or reducing wear of theprosthesis and corresponding generation of microscopic wear debrisparticles. Wear debris that may be generated will be biologically inert.Other coating materials exhibiting such biological inertness anddesirable hardness and wear characteristics may also be employed.

In addition to preventing the leaching of ions from the substrate alloyof the prosthesis, the coating will significantly improve the biologicalfixation of some substrates. More particularly, the improved biologicalcompatibility and the enhanced hardness and wear characteristicsachieved by the subject coating will define an environment that is moreconducive to bone ingrowth and hence a superior biological fixation.

The hard, abrasion resistant biologically inert coating preferably isapplied to a thickness of 5-15 microns, and more preferably in the rangeof 8-10 microns. It has been found that ceramic coatings withthicknesses in the above stated range and applied by thin filmtechnology perform without the cracking or delamination that had beenobserved with thicker coatings employed on machine tools. Furthermore,as explained below, tests have shown that articulating surfaces withcoatings having thicknesses in the above stated range remain intact andsubstantially free of damage and wear debris for more than 25 millioncycles in a test machine that accurately simulates an articulatingprosthetic joint. In contrast, a prosthetic device having the prior artcoating thickness of 2-4 microns displayed at least localized wearthrough of the coating to the substrate after only 7-8 million cycles.The virtual absence of wear and related debris on the prosthetic devicecoated to a thickness in the above stated range is believed to beattributable to the greater coating hardness that is achieved with thinfilm technology after extended ionic bombardment in the vacuum coatingchamber. Additionally, it is believed that the greater thickness gives amore crystalline structure to the ceramic coating as opposed to anamorphous structure existing with the 2-4 micron prior art coating orwith coatings applied by other coating technologies. This crystallinestructure achieved with the thicker coating provides unexpectedlygreater hardness and superior abrasion resistance. Thus, the subjectinvention enables more efficient and effective prosthetic systems.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front elevational view of a prosthesis in accordance withthe subject invention.

FIG. 2 is a cross-sectional view taken along line 2--2 in FIG. 1.

FIG. 2A is an enlargement of area A in FIG. 2.

FIG. 3A, 3B and 3C are diagrammatical cross-sectional illustrations of afemoral hip surface replacement prosthesis and the resected head of thefemur.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A femoral stem-type prosthesis in accordance with the subject inventionis identified generally by the numeral 10 in FIGS. 1, 2 and 2A. It is tobe understood that the femoral stem-type prosthesis 10 depicted anddescribed herein is only one example of the many prosthesis that mayincorporate the subject invention. The femoral stem-type prosthesis 10includes an elongated stem 12 for insertion into the intramedullarycavity 11 of the natural femur 13. The prosthesis 10 further includes acollar 14 adjacent the stem 12 and extending outwardly in anterior,posterior and medial directions therefrom. More particularly, the collaris angularly aligned and dimensioned to be in face-to-face contact witha proximal end 15 of the natural femur 13 for delivering loads thereto.A neck 16 extends from the collar 14 and terminates at a frustoconicalmounting end 17. A spherical head 18 is provided with a frustoconicalmounting cavity 19 into which the mounting end 17 of the neck 16 is fit.The spherical head 18 will define an articulating surface relative to aplastic bearing liner of an acetabular prosthetic component (not shown).Thus, the head 18 will be subjected to considerable wear as theprosthesis 10 repeatedly articulates under various applied loads. Thesmooth surfaces of the stem 12 also are subject to wear in view ofrepeated micromovement of the stem 12 relative to adjacent regions ofthe bone 13. As noted above, this wear had produced microscopic debrisin prior art prostheses.

The prosthesis 10 includes a metallic alloy substrate 20 formed from amaterial that will exhibit desirable strength, hardness and flexurecharacteristics for the particular use of the prosthesis. A preferredalloy for the substrate 20 is a titanium alloy, such as a titaniumaluminum vanadium alloy. Titanium alloys are less expensive than manyother optional alloys, are relatively easy to machine and exhibitexceptional biological compatibility. Cobalt-chromium alloys, such as acobalt-chromium molybdenum alloy may also be used as a coated oruncoated substrate 20 for at least part of the prosthesis 10.Cobalt-chromium alloys generally cost more than titanium alloys and areless biologically compatible. However, some physicians prefercobalt-chromium for at least portions of the prosthesis 10 in view ofits hardness. For example, a titanium alloy may be used as the substrate20 for the stem 12, collar 14 and neck 16. A cobalt-chromium alloy maybe used for the head 18. Other alloys selected by the physician forexhibiting appropriate strength, flexure and hardness characteristicsmay also be used for the substrate 20.

A proximal portion of the stem 12 of the prosthesis 10 is provided witha porous coating 22 for promoting bone ingrowth and achieving biologicalfixation with the bone 13 into which the prosthesis 10 is implanted. Theporous coating 22 preferably defines a continuous array of pores 24 asshown in FIG. 2A having cross-sectional dimensions of between 150-500microns each, and preferably 300-350 microns, with a preferred averagepore size of about 325 microns. The porous coating 22 may be the samealloy as the substrate 20 of the prosthesis 10 but separately appliedthereto. In other embodiments the macro surface treatment 22 on theprosthesis 10 may be defined by a mesh defining a textured or otherwiseirregular surface area that will promote bone ingrowth. In still otherembodiments the modified surface area 22 may be a knurling unitary withthe substrate 20. The particular location of the surface coating ormodification 22 on the stem 12 is described in greater detail in U.S.Pat. No. 4,904,263 which issued to the inventors herein on Feb. 27,1990. However, other locations for the surface coating or modification22 may be provided, particularly with other protheses.

As shown most clearly in FIG. 2A, the prosthesis 10 further includes ahard, wear resistant, biologically inert coating 26 applied by thin filmcoating methods uniformly over at least selected surface areas of thesubstrate 20 including at least selected surface areas that are subjectto wear. In particular, the hard wear resistant coating material 26preferably is applied to the smooth surface areas of the stem 12 thatare subject to micromovement relative to the bone and/or to the outerspherical articulating surface of the head 18. The porous coating 22 orother such surface modification of the substrate 20 for promoting boneingrowth and biological fixation may also be coated. The inert coatingmaterial 26 extends into the pores 24 defined by the porous coating 22such that the contact area between biological tissue and the porouscoating 22 of the prosthesis 10 is substantially reduced by the inertcoating 26. Thus, the leaching of ions from either the substrate 20 orthe porous coating 22 can be substantially reduced by the inert coating26 which covers all or substantial portions of the surface area of boththe substrate 20 and the porous coating 22.

The hard wear resistant inert coating 26 also preferably is applied tothe frustoconical end 17 of the neck 16, particularly for embodiment ofthe prosthesis employing a titanium alloy substrate for the stem 12 andneck 16 and employing a cobalt-chromium alloy for the head 18. In theselatter embodiments, the frustoconical cavity 19 extending into thespherical head 18 need not be coated. The hard, wear resistant inertcoating 26 applied to the frustoconical end 17 of the neck 16 achievesseveral objectives for this embodiment. First, the galvanic corrosioncaused by the interaction between abutting surfaces of the titaniumalloy neck and the cobalt-chrome alloy head is entirely avoided by theinert coating material 26 therebetween. Furthermore, the hard wearresistant characteristics of the coating material 26 substantiallyprevent the generation of any wear debris that could otherwise be causedby micromovement between the end 17 of the neck 16 and the sphericalhead 18. This enables physicians to employ the hard cobalt-chromium head18 with the less expensive titanium alloy neck 16 and stem 12 withoutthe penalty of galvanic corrosion and wear debris.

The inert coating 26 is formed from a material that is harder than thesubstrate 20. The hard, wear resistant, inert coating 26 preferably istitanium nitride or zirconium oxide, and most preferably titaniumnitride in view of its effectiveness at an acceptable cost. Otherbiologically inert materials that are now known or that may be developedmay similarly be applied for the purposes explained herein. The inertcoating 26 defines a thickness of 5-15 microns, and preferablyapproximately 8-10 microns. As noted above, it has been determined thatthe production of a coating by thin film coating technology to achieve athickness of 8-10 microns results in the ionically bonded coating beingharder than a coating of identical materials applied to the traditional2-4 micron thickness employed in the prior art. The heat and additionalion bombardment to achieve this greater thickness by thin film coatingtechnology is believed to contribute to a substantially greaterhardness. Additionally, the thicker coating 26 is believed to exhibit amore pronounced crystalline structure which is harder than the moreamorphous structure for thinner applications of the coating 26. Thisthickness also ensures that the size of the pores 24 remains within therange found to be most acceptable for promoting bone ingrowth.

The titanium nitride preferred for the coating 26, and the otheroptional inert coating materials identified above, are known to exhibitsuperior hardness, scratch resistance and lubricity. The hardness oftitanium nitride and other such coatings 26 enables nonporousarticulating surface areas of the head 18 of the prosthesis and thenon-articulating load bearing surfaces subject to micromovement to bepolished to achieve a substantially smoother surface than can beachieved with softer materials. More particularly, the articulatingsurfaces and at least certain non-articulating load bearing surfacessubject to micromovement relative to the bone are polished toapproximately 1.0 microinch roughness or less. This contrasts to priorart prostheses which have been polished only to about 4.0 microinchroughness. The much smoother characteristics of surfaces subjected towear that are enabled by the harder coating 26 contribute substantiallyto wear resistance, abrasion resistance and lubricity. The hard smoothsurface achieved by the coating 26 on the head 18 substantiallyminimizes the generation of metallic wear debris that could otherwiseresult from the metal/plastic articulation resulting from engagement ofthe head 18 of with an acetabular prosthetic component (not shown). Thecoating thickness of approximately 8-10 microns on the articulatingsurface of the head 18 has remained substantially in tact onarticulating surfaces after subjection more than 25 million cycles in atest machine where the head 18 of titanium alloy with the coating 26 oftitanium nitride is engaged with and articulated against an acetabularprosthetic component having a plastic bearing liner. The more than 25million cycles without significant damage or wear debris issubstantially in excess of the performance enabled by prior artarticulating prosthetic joints coated to 4 microns thickness where awear through of the coating to the substrate was observed after 7-8million cycles. The superior performance of the thicker coating isbelieved to be attributable to the greater hardness that is unexpectedlyachieved with the thicker coatings applied by thin film coatingtechniques as explained herein. Additionally, none of the purportedproblems of cracking and delamination reported in the machine tool artwere observed after 25 million cycles on the prostheses with the 8-10micron thick coating.

The inert coating 26 is applied after fabrication, polishing andpreliminary cleaning, by initially fixing the prosthesis 10 on arotating mount in a vacuum coating chamber. Following evacuation of thechamber, ionic surface cleaning is achieved by striking an arc andproducing a highly ionized titanium plasma. The prosthesis 10 are givenhigh negative charge which attracts the plasma and subjects theprosthesis 10 to a high energy titanium ion bombardment. In addition toionic surface cleaning, this also deposits a thin titanium film on theprosthesis 10 and heats them to the requisite temperature for TiNcoating. When a titanium alloy prosthesis is coated, this thin titaniumfilm is integrated into the titanium substrate 20. On other materialsthis film is ionically bonded to the substrate 20 and protects theprosthesis 10 against surface corrosion. Nitrogen gas at low partialpressures is then introduced into the chamber. This gas reacts with thetitanium plasma and produces the ionic deposition of a highly-adherentceramic TiN coating 26 about 8-10 microns in thickness.

An alternate embodiment of the prosthesis is illustrated in FIGS. 3A-3C.More particularly, a hip surface prosthesis is identified by the numeral40 in FIGS. 3A-3C and includes a highly polished spherical outer loadreceiving surface 42 and a porous coated interior primary surface 48having a longitudinally extending stem 58 projecting centrally from theinterior of the prosthesis 40. The prosthesis 40 is for being implantedin the resected head 44 of the femur 46 with the stem 58 being driveninto a prepared hole 62 formed in the neck 60 of the femur 46. The outerapplied load receiving surface 42 of the prosthesis 40 is for replacingthe articular surface of the head of the natural femur and for receivingthe load applied to the prosthesis during articulation. The porouscoated interior surface 48 is intended to provide interlocking fixationbetween the prosthesis 40 and the resected head 44 of the femur 46 bydirect bone ingrowth. As explained with the previous embodiment, theporous coating 48 results in a substantial increase in surface area. Toprevent an unacceptable ion release from the porous coating 48 and toenhance bone ingrowth, the porous coating 48 is provided with a topinert coating 66 at a thickness of 5-15 microns and preferably about8-10 microns. The inert coating 66 preferably is titanium nitride, butmay be one of the alternate coatings identified above. The inert coating66 also is preferably applied to the outer applied load receivingsurface 42 and to the stem 58. The inert coating 66 applied to theapplied load receiving surface 42 is polished to achieve an acceptablyhigh degree of smoothness. The hardness of the inert coating 66substantially prevents the generation of metallic wear debris that wouldotherwise result from articulation between the surface 42 and theacetabular component of a prosthetic system. Similarly, the hardness ofthe inert coating 66 applied to the stem 58 prevents the generation ofwear debris that could result from either the initial driving of thestem 58 into the prepared hole 62 in the neck 60 of the femur 46 or frompost-operative movement resulting from loads applied to the prosthesis.

While the invention has been described with respect to a preferredembodiment, it is apparent that various changes can be made withoutdeparting from the scope of the invention as defined by the appendedclaims. In particular, the coating may be defined by other biologicallyinert and sufficiently hard metals or ceramics. The coating may beapplied to prosthesis other than the femoral prosthesis depicted in theFigures. The coating may be applied to the entire prosthesis and notonly to areas thereof having the bone ingrowth surface modification orcoating. The surface treatment to which the coating is applied may beany surface irregularity employed for promoting bone ingrowth andnecessarily increasing the surface area of the prosthesis. These andother variations will be apparent to the person skilled in the art.

We claim:
 1. An orthopedic prosthesis formed from a metallic alloy andhaving at least one load bearing surface movably disposed adjacent to anopposed surface and at least one bone ingrowth surface, said prosthesiscomprising a substrate formed from at least one selected metallic alloyand an abrasion resistant, biologically inert coating formed from aceramic material harder than the substrate, said coating being ionicallybonded to the substrate forming a unitary layer on portions of theprosthesis defining the load bearing surface and portions of theprosthesis defining the bone ingrowth surface, said coating defining athickness of 8-10 microns.
 2. An orthopedic prosthesis as in claim 1,wherein the coating defines a crystalline structure.
 3. An orthopedicprothesis as in claim 1, wherein at least a portion of the substrate isformed from a titanium alloy, and wherein the ceramic coating comprisestitanium.
 4. An orthopedic prosthesis as in claim 3, wherein the coatingis titanium nitride.
 5. An orthopedic prosthesis as in claim 3, whereinat least a portion of the prosthesis is formed from a cobalt-chromiumalloy.
 6. An orthopedic prosthesis as in claim 5, wherein the portion ofthe prosthesis comprising the cobalt-chromium alloy is engaged with aportion of the prosthesis having the titanium alloy substrate with thecoating thereon.
 7. An orthopedic prosthesis as in claim 5, wherein theportion of the prosthesis formed from the cobalt-chromium alloy is notcoated with the ceramic material.
 8. An orthopedic prosthesis as inclaim 1, wherein the load bearing surface comprises a non-articulatingsubstantially smooth load bearing surface disposed for load bearingmicromovement adjacent to a bone.
 9. An orthopedic prosthesis as inclaim 8, wherein the load bearing surface further comprises anarticulating surface disposed for load bearing articulation with respectto another prosthetic component.
 10. An orthopedic prosthesis as inclaim 9, wherein the bone ingrowth surface area comprises a non-smoothsurface configuration for increasing surface area and promoting boneingrowth.
 11. An orthopedic prothesis as in claim 1, wherein the coatingformed on the load bearing surface is polished to a smoothness ofapproximately 1 microinch.
 12. A prosthetic device for biologicalfixation to a bone, said prosthetic device comprising a metallic alloysubstrate, a first portion of said substrate defining a bone ingrowthsurface area having a non-smooth surface configuration for increasingsurface area and promoting bone ingrowth, a smooth articulating surfacearea spaced from the bone ingrowth surface area and a smooth loadbearing surface area spaced from both the bone ingrowth surface area andthe articulating surface area, the bone ingrowth surface area, thearticulating surface area and the load bearing surface area all beingprovided with a unitary substantially biologically inert scratchresistant coating exhibiting greater hardness than the metallic alloysubstrate for substantially preventing metallic ion release from thesubstrate and for preventing production of metallic wear debris, saidbiologically inert scratch resistant coating defining a thickness of8-10 microns, whereby the coating prevents ion leaching from thesubstrate into adjacent tissue.
 13. A prosthetic device as in claim 12,wherein the prosthetic device is a femoral stem-type prosthetic devicehaving a stem defining a portion of the prosthetic device having saidbone ingrowth surface area, a neck extending from said stem and a headhaving a cavity therein engaging a proximal portion of said neck, thehead comprising the articulating surface of the prosthetic device, theproximal portion of the neck having the coating thereon and the cavityin the head being substantially free of the coating.
 14. A prostheticdevice as in claim 12, wherein the substrate is formed from a titaniumaluminum vanadium alloy.
 15. A prosthetic device as in claim 12, whereina portion of the prosthetic device is formed from a cobalt-chromiummolybdenum alloy.
 16. A prosthetic device as in claim 12, wherein thebiologically inert coating is a titanium nitride.
 17. A prostheticdevice as in claim 12, wherein the biologically inert coating iszirconium oxide.
 18. A prosthetic device as in claim 12, wherein thebone ingrowth area is defined by a plurality of pores each of whichdefines a cross-sectional dimension in the range of 150-500 microns. 19.A prosthetic device as in claim 12, wherein the biologically inertmaterial is selected from the group consisting of titanium nitride,zirconium, titanium boride, titanium carbide, aluminum oxide anddiamond.
 20. A prosthetic device as in claim 12, wherein the prostheticdevice comprises a first component having the smooth load bearingsurface thereon and a second component having the smooth articulatingsurface thereon, the first and second components having engagementsurfaces defining an interface therebetween, said first component havingthe coating on the engagement surface, the second component beingsubstantially free of the coating on the engagement surface thereof. 21.An orthopedic prosthesis comprising at least one substantially smoothnon-articulating load bearing surface area for placement in proximity toa bone and at least one substantially smooth articulating load bearingsurface area for articulation against another orthopedic prostheticcomponent, the orthopedic prosthesis comprising a substrate formed froma titanium alloy and a coating formed from a ceramic material comprisingtitanium ionically bonded to at least the non-articulating load bearingsurface and the articulating load bearing surface of the orthopedicprosthesis to a thickness of 8-10 microns, whereby the ceramic coatingdefines a hardness greater than the substrate and substantially preventsabrasion of the substrate.